In a future mobile communication system, e.g., a Beyond Third Generation (B3G) system or a Long Term Evolution-Advanced (LTE-A) system, higher peak data rates and cell throughput will be offered and also a larger bandwidth will be required, but at present there are few unallocated bandwidths below 2 GHz, so a part or all of the bandwidth required for the B3G System has to be sought in a higher frequency band, e.g., above 3 GHz. The higher the frequency band is, the faster an electric wave will be attenuated, and the shorter distance it will travel over so a larger number of Evolved NodeBs (eNBs) are required to ensure seamless coverage in the same area, which will undoubtedly increase network deployment cost due to the typically expensive eNBs. In order to address the issues of network deployment cost and coverage, various manufactures and standardization organizations come to research the introduction of a Relay Node (RN) to a cellular system for coverage improvement.
FIG. 1 is a network architecture of the LTE-A system with an RN deployed, where the RN is wirelessly connected with a Donor Evolved Node B (DeNB) which functions as a proxy. The RN accesses to core network via a donor cell managed by the DeNB without a direct wired interface with the core network, and each RN can control one or more cells. In this architecture, an interface between a User Equipment (UE) and the RN is referred to as a Uu interface, and an interface between the RN and the DeNB is referred to as a Un interface.
An X2 handover is divided into three phases comprising handover preparation, handover execution and path switch (i.e., handover completion) as illustrated in FIG. 2. The handover preparation phase is initiated by a source eNB which select a target eNB. A handover request acknowledgement message carries a handover command message generated by the target eNB and is forwarded to the UE by the source eNB, as illustrated in FIG. 3.
In a relay scenario, the DeNB reads an identifier of a target cell in a handover request message and forwards the handover request message to the corresponding target eNB after receiving the handover request message.
In a handover procedure, after selecting the target cell, in order to increase the success rate of the UE reestablishment to another cell, the source eNB can further prepare Radio Resource Control (RRC) reestablishment information of other cells served by the target eNB besides the target cell and include the prepared RRC reestablishment information in an RRC context Information Element (IE) in the handover request for an X2 handover or in a Source To Target Container IE of the handover required message for an S1 handover.
In order to increase the handover success rate, when the source eNB prepares the RRC reestablishment information of the target cell, it can prepare reestablishment information of cells served by multiple target eNBs in parallel, and generate multiple handover request messages and send the multiple handover request messages to the corresponding target eNBs. Thus if the UE fails to handover to one of the eNBs, then when it tries to reestablish to another eNB, the reestablishment procedure is very likely to succeed if this eNB has previously received a handover request. Multiple handovers in parallel can increase the handover success rate of the UE when there are more overlapping coverage areas among several eNBs.
The RRC reestablishment information of a cell generally includes a key and an integrity check code associated with the cell. When the UE selects a specific cell to perform RRC reestablishment, it will report an integrity check code corresponding to the cell, and a target eNB compares the integrity check code reported by the UE with its own stored integrity check code and can allow an access of the UE to the cell if the integrity check codes are consistent.
In the prior art, the source eNB need not prepare reestablishment information of its own cell but will calculate an integrity check code and verify the UE only if the UE perform RRC reestablishment to its own cells.
The inventors have found during making of the invention the following technical problems in the prior art:
In the relay scenario, for a handover of the UE from the RN to the target eNB, if the handover of the UE fails, then the UE is very likely to select a cell of the DeNB serving the RN to perform reestablishment and even possibly select a cell of another RN served by the DeNB to perform reestablishment, but the DeNB has no RRC reestablishment information of the DeNB cells and the other RN served by the DeNB has no RRC reestablishment information of its own cells, so the reestablishment may fail when the UE selects one of these cells to perform reestablishment.
Similarly, for a handover of the UE from the source eNB to the RN, when the handover fails, if the UE selects a cell of the DeNB serving the RN or a cell of another RN served by the DeNB to perform reestablishment, and the DeNB has no RRC reestablishment information of the cell managed by itself and the other RN served by the DeNB has no RRC reestablishment information of the cell managed by itself, then the cell reestablishment may fail.